There is currently enormous interest in the dark side of oxygen, i.e., its capacity to cause disease. Hyperoxia is causal in disease processing such as retrolental fibroplasia and adult respiratory distress syndrome. Oxygen radicals formed under normoxic conditions have been implicated in diverse pathologies associated with inflammation, aging, drug toxicity, shock, reperfusion injury (i.e., that damage occurring after ischemia is relieved), radiation damage, and cancer. Oxidant damage is thought to be due to generation of partially reduced oxygen species such as superoxide (0.sub.2.sup.-.) and hydrogen peroxide (H.sub.2 0.sub.2) which give rise to more reactive species such as hydroxyl radical (OH.sup..), all of which cause damage to cellular components like nucleic acid, protein, lipids, and carbohydrate.
Although there is not unanimity on the pathogenic mechanism of 0.sub.2 toxicity, much current thinking implicates 0.sub.2.sup.-. and H.sub.2 0.sub.2, perhaps involving transition metals within the cell, in production of highly reactive radicals. For example, Fenton chemistry is part of an "iron-catalyzed Haber-Weiss reaction" ##STR2## that leads to the production of hydroxyl radical, so reactive that it attacks proximate molecules indiscriminately within a few molecular diameters. The net effect EQU O.sub.2.sup..- +H.sub.2 0.sub.2 .fwdarw.OH.sup.. +OH.sup.- +0.sub.2
may be responsible for pathology. The question then becomes how partially reduced oxygen intermediates such as O.sub.2.sup..- and H.sub.2 0.sub.2 are or can be maintained at low enough intracellular levels to minimize such interactions and the resultant damage. One mechanism is to prevent their production. The primary cellular oxygen consumer, the respiratory chain, has cytochrome oxidase as the terminal electron acceptor which only releases oxygen upon its complete reduction to water. To remove reactive 0.sub.2 intermediates, catalases are present to deal with H.sub.2 0.sub.2, especially when it is produced in or near peroxisomes. Glutathione peroxidase (see reaction [10], infra) acts as a major scavenging enzyme for cytosolic H.sub.2 0.sub.2 and certain other hydroperoxides, with oxidized glutathione (GSSG) reduced by glutathione reductase (reaction [5], infra). Superoxide dismutase (SOD) exists in most aerobic cells to catalyze EQU 20.sub.2.sup..- +2H.sup.+ .fwdarw.0.sub.2 +H.sub.2 0.sub.2 [ 4]
oxidation of superoxide. Much current effort implicates superoxide dismutase as a key protectant from oxidative stress, a hypothesis that has been supported by the discovery of mutants of E. coli and yeast that lack superoxide dismutase and have specific lesions in aerobic growth. Other cellular protecting agents such as thiols, vitamin E and ascorbic acid have less clearly defined roles.
The belief that oxygen radicals are responsible for disease has prompted attempts at therapy with superoxide dismutase and catalase in ischemia/reperfusion injury, inflammation, radiation injury and other disorders, with reports of success, although not all of the studies were carefully controlled and double blind in nature. The idea behind these approaches is that by interfering with the production of reactive oxygen species one could interfere with the pathophysiology. Despite its attractiveness, there are several difficulties with this approach, both conceptionally and practically. The pathogenesis of disease by oxidative stress and the exact role of different reduced 0.sub.2 species are not understood. Moreover, it is difficult to deliver proteins such as superoxide dismutase and catalase to the appropriate locus, and it is unlikely that they will reach the cytosolic compartment even when administered parenterally. Attempts have been made to produce low molecular weight superoxide dismutase analogues, such as Cu-salicylates or other chelated transition metals that might enter target cells and avoid the immunologic challenge posed by administering foreign proteins parenterally.
A principal reason that it has been difficult to unambiguously implicate reactive oxygen species in the pathogenesis of specific diseases is the absence of experimental models where the enzyme is actually present at the cellular target. Moreover, much of the suspect pathology occurs over a protacted period of time (e.g., in aging, carcinogenesis, etc.), so that linking slight modifications in reactive oxygen intermediates with disease is difficult.